1. Field of the Invention
The present invention relates to manufacturing of an active matrix display panel including a display element and a thin film transistor (hereinafter referred to as TFT) which controls connection between the display element and a power source line, and more particularly to processing related to a defective pixel.
2. Description of Related Art
Organic electroluminescence (hereinafter referred to as “EL”) display panels have been conventionally known as one type of flat display panel. Unlike liquid crystal display panels (LCDs), organic EL display panels are self-emissive. This feature, as well as their brightness and clarity, has fueled expectations of their widespread use.
Organic EL displays are composed of a great number of organic El elements, which function as pixels, arranged in a matrix. Further, of passive and active driving methods which can be used for organic EL elements as well as for LCDs, as is the case with LCDs, the active matrix method is also preferable for driving an organic EL element. More specifically, the active matrix method, in which a switching element provided for each pixel is controlled so as to control display of each pixel, is preferable because a higher resolution image screen can be realized than with the passive method, in which a switching element is not provided for each pixel.
An LCD display uses a single switching element (TFT), which is connected directly to a pixel electrode, whereas an organic EL panel uses two TFTs and one capacitor. FIG. 3 shows an example structure of a pixel circuit in an organic EL panel which uses conventional thin film transistors (TFTs). An organic EL panel is configured by arranging such pixels in a matrix.
Specifically, to a gate line extending in the row direction, a gate of a first TFT 10 which is an n-channel thin film TFT to be selected by the gate line is connected. A drain of the first TFT 10 is connected to a data line DL extending in the column direction and a source of the first TFT 10 is connected to one end of a storage capacitor CS. The other end of the storage capacitor CS is connected to a capacitor line SL which is a low voltage power source. Further, a connection point between the source of the first TFT 10 and the storage capacitor CS is connected to a gate of a second TFT 40 which is a p-channel thin film transistor. A source of the second TFT 40 is connected to a power source line VL and a drain of the second TFT 40 is connected to an organic EL element (EL). The other end of the organic EL element (EL) is connected to a cathode power source CV.
Accordingly, when the gate line GL is at H level, the first TFT 10 turns on, and the data then on the data line DL is stored in the storage capacitor. The current of the second TFT 40 is then controlled in accordance with the data (charge) stored in the storage capacitor CS, and current in accordance with the current of the second TFT 40 flows in the organic EL element EL, which then emits light.
While the first TFT is turned on, a video signal corresponding to that pixel is supplied to the data line DL. Thus, the storage capacitor CS is charged in accordance with a video signal supplied to the data line DL, whereby the second TFT 40 causes corresponding current to flow for controlling the brightness of the organic EL element EL. In other words, by controlling the gate potential of the second TFT 40 for controlling current to be supplied to the organic EL element, gray scale display for each pixel is performed.
In an organic EL panel as described above, the first TFT or the second TFT provided for each pixel may become defective. When these first and second TFTs have a defect, there is a possibility that a pixel is displayed as a luminous dot (a pixel which never becomes dark) or a dark dot (a pixel which never becomes bright), or that the data line DL connected to these TFTs is affected by generation of a short circuit, thereby causing a defect in the line. Accordingly, such a detective portion of the first TFT1 or the second TFT2 is conventionally separated from a line of connection so as to repair and normalize the pixel.
Although such a repair method results in a number of dark dots, this does not in itself significantly affect the usefulness of the organic EL panel as a product. In fact, a significant increase in production yields can be achieved by simply darkening luminous dots.
The above repair can be performed by disconnecting a line extending to the defective portion. More specifically, as in the case of LCDs, a line connecting the second TFT2 and the power source line or the pixel electrode may be disconnected using laser irradiation such as YAG laser.
In this manner, the defective portion can be separated from the lines and can be darkened, so that the problem concerning the overall display can be solved.
The repair treatment using YAG laser, however, results in formation of a deep hole in the panel. For example, when the line disconnection using YAG layer is performed in a stage where the TFTs are formed for each pixel, layers formed below the line is also blown off, where a deep hole is created. While organic layers and a cathode are to be formed subsequently in the case of an organic EL panel, these layers are too thin to sufficiently cover the hole. As a result, side surfaces of the organic layers of the organic EL element are also directly exposed to a space above the cathode, which may accelerate deterioration of the organic layers due to intrusion of moisture and may increase defective pixels.
In LCDs, when line disconnection is performed using a laser, as described above, not just the line but also other layers are removed and a hole is created where laser light is applied. As a result, side surfaces of each layer are exposed, often resulting in rapid deterioration and alignment disorders in a fine pixel device.